We have studied antiferroelectricity in liquid crystals from various viewpoints and established the P_X model ; the hindered rotational motion of molecules about their long axs produces spontaneous polarizations parallel to the tilt plane near smectic layr boundaries and stabilizes the antiferroelectric SC_A^<**> phase. Based on the P_X model, we have analyzed the frustration between ferro- and antiferro-electricity, introduced a novel concept, thresholdless antiferroelectricity (TLAF), and explained the V-shaped switching observed in a novel, apparently antiferroelectric liquid crystal mixture and usable in liquid crystal displays (LCDs) with active matrix electrodes. Details are :1) Higher-order X-ray Bragg peaks indicated a correlation between the stable antiferroelectricity and higher smectic-layr order.2) Spectroscopy clarified the following four facts :(1) Polarized Raman confirmed the molecular arrangements in SC^<**> and SC_A^<**>.(2) NMR and polarized IR (PIR) verified the ben
… Moret structure, the precessional motion and the biaxiality around the average chain axs of chiral alkyl chains in liquid crystal phases.(3) PIR proved the hindrance of molecular rotational motion about their long axs and a difference in the hindered direction between SC^<**> and SC_A^<**>, assigning the threshold of the triastable switching to the barrier between the SC^<**> and SC_A^<**> hindered directions and suggesting the P_X model.(4) PIR verified the chiral alkyl chain precessional motion, and NMR indicted its collective character.3) The optical Kerr effect with 130 fs pump pulses dose not show any critical slowing down, indicating no coupling between the laser-induced molecular reorientation with tau-10^<11>s and the polarization.4) Optical microscope observation of the subphases in free-standing films under temperature gradients supported Yamashita's ANNNI+J_3 model.5) Electro-optical observation of an apparently antiferroelectric liquid crystall mixture showed the V-shaped switching, resulting in the TLAF based on the randomization and Langevin type alignment of director tilting from layr to layr.6) PIR and NMR showed that the P_X model also holds in SI_A^<**>.7) CD spectroscopy proved the dynamical character of the SC_<alpha> helicoidal structure, supporting a speculation of SC_<alpha> characterized by thermally excited C-director disclinations.8) Dimer molecules with chiral units connected with polymethlene chain indicated the importance of dipole-dipole interaction in SC_A^<**>.9) A 5 T superconducting magnet simplified the fabrication of liquid crystal cells, furthering to study the aligning mechanism and the defect structure. Less